The present invention relates to devices which stack containers. In particular, it relates to devices which stack a plurality of containers or products which are self-contained, and which are capable of being continuously delivered.
Many products such as food, automobile parts, books, tools, video cassettes, and health care products, for example are packaged in substantially cubical (often rectangular) containers before being stored and shipped to wholesalers and retailers. Because such products are produced in large volume, it is desirable to handle them in bulk--as by stacking or otherwise arranging them in larger containers or cases.
There are three basic types of automated devices, commonly referred to as container stackers, known which are used to stack containers. One known automated stacker is referred to in the art as a cross-pusher. In these devices, containers are fed vertically downward through a chute in a single row onto a substantially horizontal container receiving surface. A substantially vertical plate (cross-pusher) contacts one or more of the fed containers and pushes them in a horizontal path toward a second plate. The second plate is also substantially vertical and perpendicular to the container receiving surface. A pneumatic cylinder attached to the second plate places pressure on each container being stacked. The force is delivered in a direction opposite the direction of movement of the containers.
A pneumatic cylinder moves the first plate and cross-pusher along the container receiving surface (retracting after each container) until the engaged containers contact either the second plate or another container. After a selected number of containers have been pushed across the container receiving surface, the cross-pusher retracts leaving a formed block of containers.
Cross-pusher stackers are compact, and are relatively simple in design. However, the stacker does not maintain enough control of the containers to operate at high speeds--exceeding 200 containers per minute, for example. For example, the containers stacked above the container receiving surface (on another container) are unstable and are known to fall out of the desired alignment, causing the entire stacking line to become jammed. The use of a cross-pusher type case stacker in some applications is known to cause production delays due to jamming.
Even if groups of containers are stacked into a single layer of containers, as the first plate withdraws to allow the next container fall, containers tend to fall in a direction opposite the direction of horizontal travel and jam the case stacker. Further, a cross-pusher stacker is known to damage containers, because the containers drop in an uncontrolled fashion. The rough handling characteristics of such a stacker limit its use to products which are not fragile. It would be inappropriate for example to select such a stacker to stack containers of lasagna noodles because the uncontrolled fall would fracture the noodles.
Another known case stacker is a star wheel type stacker. A continuous stream of containers are fed horizontally onto a star shaped wheel. The containers are fed in a single stream. The wheel has a number of vees, each of which are shaped to receive one container. The containers are oriented in the feed stream to lie with the longest dimension being substantially horizontal and in the direction of flow, and the shortest dimension being substantially vertical. The width of the star wheel, including the width of each vee is selected to be substantially the same as the width of each container, as measured transverse to the direction of travel of the feed.
The horizontal feed stream passes through a pair of oppositely spaced pinch belts which uniformly deliver a continuous stream of containers to the wheel. The pinch belts are driven and this provides the power that drives the product to the star wheel. In most cases the wheel is simply turned by the force of the incoming products.
The feed rate is coordinated with the linear speed of the star wheel at the location of the vees. Each container comes into contact with a vee, the vee located along the outer edge of the rotating wheel. The vee is shaped such that as the wheel rotates, the containers become substantially vertically oriented, and are gently placed onto a horizontal container receiving surface.
After a selected quantity of containers are collected, the group is held together by a pair of vertical plates placed on opposite sides of the container receiving surface. The containers are then upstacked.
Although this type of stacker is suitable for higher speed lines, e.g.--exceeding 200 containers per minute, and provides for gentle product handling, it is more costly to produce and purchase, and requires a great deal of space for its operation. Maintenance costs are higher as the construction is more complex, as compared to the cross-pusher, and more downtime is necessary for repair. Another disadvantage of the star wheel design is that the design does not provide a means to apply tension to the containers while the containers are located in the vees. Loose containers become misaligned, and jam the line causing further downtime.
A third type of known device, often referred to as an orbital accumulator, includes a single substantially vertical push bar which is pivotally attached at an upper and lower point to upper and lower wheels. Both wheels are located on a substantially vertical axis in a vertical plane and are of substantially the same diameter. Both wheels are driven by a single motor, the motor driving a sprocket connected by a shaft to the lower wheel. The upper wheel is connected by a shaft to a driven sprocket. As the motor shaft rotates, the drive sprocket, driven sprocket and chain connecting the sprockets drive the upper and lower drive wheels in unison.
The vertical push bar is pivotally attached to a point on each wheel. The distances between the pivot pin on the wheel and the center of rotation are equal for both wheels. The pivot point locations are selected so that the push bar remains substantially vertical, while the apparatus is in operation. The upper surface of the push bar moves in a substantially circular path.
The upper surface of the push bar contacts a lower surface of the container being fed into the accumulator. The circular path pattern has certain disadvantages. Most importantly, the speed of the push bar has both a horizontal and vertical element during operation. When the front surface of the push bar contacts the container, the push bar slaps the container because there is a substantial horizontal speed component at the point of impact.
Another disadvantage of such an orbital accumulator is that the diameter of the wheels must be selected in order to achieve the proper amount of drop of the push bar. The upper surface must become flush with the container receiving surface, or drop even lower in order to allow the containers to slide horizontally. If it is desired to stack a box which is taller than it is wide, the amount of drop must be at least equal to the height of the box. Because the path of travel is fixed, the pusher advances the boxes the same horizontal distance, causing a large distance to form between the newly vertically dropped box and the newly advanced box. Allowing such a large distance to form causes boxes to become misaligned, and the stacking line can jam.
Although this type of device has the advantage of being compact, and inexpensive to produce, it is limited in both speed and flexibility. The horizontal travel is limited. It is not possible to obtain the necessary vertical reach to engage more than one product in a single stroke. Since the device is limited to pushing one container at a time, the capacity of the device is also limited. The geometry of the device is limited to use with small products, and the movement is harsh on the products.